This section is intended to provide a background or context to the embodiments disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
Time Domain Crosstalk (TDX) is used to diagnose faults in multi-conductor twisted-pair cabling. The measured TDX can be used to display the crosstalk between pairs versus time. Because the signal propagation velocity is known, the time location of a crosstalk can be converted to a distance, and the crosstalk's position along the length of the cable may be determined.
The stimulus and data collection for the TDX can be done directly in the time domain, but it can also be derived from frequency-domain measurements via FFT (Fast Fourier Transform) or other transform techniques. Frequency domain measurements can typically be performed with higher performance and less expensive hardware. Additionally, frequency domain data may be needed for standards-compliant testing of near-end crosstalk (NEXT) such that the TDX can exploit data from a NEXT measurement sweep.
The TDX may be generated from frequency domain measurements. First, the near-end crosstalk (NEXT) is preferably measured at a number of discrete frequencies. To do this, a single-frequency sinusoidal source voltage is injected into a wire pair at one end of the cable. The resulting voltage on a second wire pair at the same cable end is measured for magnitude and phase. It is noted this second wire pair may be referred to as the victim pair. The complex ratio of the voltage on the victim pair to the voltage on the source pair is the NEXT for that combination of pairs. The NEXT measurement may be repeated for a large number (typically hundreds) of frequencies. All these measurements are preferably collected into a measurement vector.
A plot of the results of a NEXT measurement as a function of frequency (curve 10) is shown in FIGS. 1A and 1B where curves 10 and 10a are respectively extending from zero to 800 MHz. A curve 12 in both FIGS. 1A and 1B represent an example of an industry standard test limit extending from zero to 500 MHz. A vertical scale in FIGS. 1A and 1B represents a loss in the NEXT measurements using positive dB units. With reference to FIG. 1A, what is shown is a NEXT margin between the curves 10 and 12 as being relatively small and distinctly positive only below 200 MHz. In FIG. 1B the NEXT margin between the curves 10a and 12 is relatively larger than in FIG. 1A and distinctly positive below 500 MHz, such that the worst margin occurs at high frequencies near 500 MHz.
Typically, a next step is to window the measurement. It is to be appreciated windowing is an element-wise multiplication of the measurement vector by a same-length window vector. Without windowing, the measurement vector contains a step discontinuity at one or both ends of the frequency range resulting in confusing Gibbs' phenomenon oscillations after transformation to the time domain. The windowing smoothly tapers the end of the measurement vector to eliminate the step discontinuities. A plot of a conventional smoothing low pass window function 14 (using arbitrary units) as a function of frequency is shown in FIG. 2.
After windowing, the windowed data is transformed to the time domain to create a TDX plot. The transformation may be accomplished by a conventional method such as an Inverse Fourier Transform (IFFT). An alternative to the IFFT for this application, which also compensates for the cable's attenuation and dispersion (when different frequency components propagate through the cable at different speeds), is a method taught in U.S. Pat. No. 7,295,018 using a loss and distortion technique which is incorporated herein by reference in its entirety. An example of a curve 16 depicting dependence of a TDX (generated by transforming the windowed data) in units 10′V/V (on vertical scale) as a function of a sample point in time domain is shown in FIG. 3. A prominent peak 18 appears near the 30th sample point (in time domain). This may be caused by a localized source of crosstalk at that location in the cable.